In offshore oil production, marine drilling vessels such as offshore platforms/drilling rigs or drillships are used for exploratory offshore drilling and for drilling offshore wells. Several technical difficulties are involved in offshore drilling.
One difficulty is the requirement of the vessel to precisely keep its position above the drilling site at the seabed during a drilling operation. Modern vessels comprise dynamic positioning systems which use several sensors for position determination. This information is then used to independently control several thrusters of the vessel so that horizontal movements relative to the seabed are minimized. An offshore drilling rig may for example comprise 6-8 azimuth thrusters allowing a precise positioning of the vessel. The thrusters are operated continuously when the dynamic positioning system is active.
A further difficulty is the vertical movement (heave) of the marine drilling vessel. The heave motion of the vessel is caused by sea waves and changes the distance between the vessel and the seabed. In a drilling operation, this motion causes the weight on the drill string to change. Conventional drilling vessels were thus only able to drill in calm seas.
To reduce the amount of downtime and enable the drilling even when larger waves prevail, passive and active heave compensation were introduced. In heave compensation, the weight on the drill bit (WOB, weight on bit) at the bottom hole assembly of the drill string is kept as constant as possible. This is achieved by allowing the drill string to move relative to the drilling vessel such that the drill string remains almost stationary relative to the seabed (e.g. a relative movement of only several centimeters).
The drill string can for example be raised and lowered relative to the vessel by means of heave compensating drawworks. The drawworks comprises a spool or winch drum driven by electric motors for paying in and out of drill line. The drill line is used to raise and lower a travelling block to which the drill string is attached. For heave compensation, the drill line is reeled in and out periodically to achieve an almost constant weight on bit. When lowering the drill string, the electric motors need to be slowed down. This is achieved by burning up electric energy produced by the motors in water cooled braking resistors. When raising the drill string, the electric motors of the drawworks require a substantial amount of electric power to be operated.
It is desirable to reduce the energy consumption of the drilling vessel. Further, the water cooled braking resistors are heavy and require a substantial amount of space, while not having any further useful function. It is also desirable to avoid peaks in the power consumption on the drilling vessel, as such peaks make the energy production by means of generators less efficient (as the generators need to operate outside the most efficient range) and furthermore require the power supply of the drilling vessel to be designed for larger loads.